This invention relates to a system for detecting a misfire condition in a multicylinder internal combustion engine.
If a cylinder misfire condition should occur during operation of an internal combustion engine that includes a catalytic converter to promote oxidation of hydrocarbon and carbon monoxide gases present in the exhaust gases, the unburned fuel that is discharged into the exhaust passage from the misfiring cylinder greatly increases the reaction temperature of the converter to an extent that may lead to structural failure. This is particularly the case in a system in which secondary air is supplied to the exhaust passage upstream of an oxidizing converter in order to provide an oxidizing atmosphere to promote the catalytic oxidation of the unburned hydrocarbon and carbon monoxide gases.
Typically, engine emission control systems include a three-way catalytic converter either alone or upstream from an oxidizing converter in the exhaust passage. The three-way catalytic converter accomplishes both the oxidation of carbon monoxide and hydrocarbon gases and reduction of nitrogen oxides when the air/fuel ratio of the mixture supplied to the engine is maintained at substantially a stoichiometric ratio. To maintain this required air/fuel ratio for three-way catalytic conversion, closed loop air/fuel ratio control systems are employed which include an oxygen sensor, such as a catalytically coated zirconium dioxide sensor, which monitors the exhaust gases discharged from the engine. This type of sensor behaves substantially as a switch and provides a bilevel signal indicating the rich or lean condition of the exhaust gases relative to a stoichiometric ratio. A controller typically including an integral control term monitors the output of the oxygen sensor and adjusts the air/fuel ratio of the mixture supplied to the engine in a direction to obtain a stoichiometric ratio.